1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated continuously variable transmission of belt-and-pulley type used for a motor vehicle.
2. Discussion of the Related Art
A belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable is known as a transmission incorporated in a power transmitting system for a motor vehicle. This continuously variable transmission includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys for transmitting power therebetween, and a pair of hydraulic actuators for changing effective diameters of the pulleys. An example of a hydraulic control apparatus for such a belt-and-pulley type continuously variable transmission is disclosed in co-pending U.S. patent application No. 07/647,424, now U.S. Pat. No. 5,157,992. In the hydraulic control apparatus disclosed in the co-pending application, a suitably regulated line pressure of the hydraulic system is applied as a belt tensioning pressure to one of the hydraulic actuators, so as to control the tension of the belt of the transmission.
The hydraulic control apparatus of the above type utilizes an angle of opening of a throttle valve disposed in a suction pipe of the engine, for example, as a parameter representative of a currently required output of the engine. To this end, the control apparatus has a throttle sensing valve which includes: a cam which is mechanically connected to the throttle valve or a member moving with the valve, through a link mechanism such as a cable or other link member, and which is rotated as the throttle valve is opened; and a plunger which engages the cam, and is axially moved by a distance corresponding to an angle of rotation of the cam. Thus, the throttle sensing valve is adapted to produce a throttle pressure corresponding to the opening angle of the throttle valve, based on the distance of the axial movement of the plunger. The above-indicated belt tensioning pressure is regulated by a second pressure regulating valve (belt tensioning pressure regulating valve), based on the throttle pressure generated by the throttle sensing valve, so that the belt tensioning pressure increases with an increase in the opening angle of the throttle valve.
However, the throttle sensing valve has an output characteristic (as indicated in FIG. 6) that the throttle pressure increases with the opening angle of the throttle valve at a comparatively high rate while the opening angle is relatively small, and at a comparatively low rate while the opening angle is relatively large. Further, the cam of the throttle sensing valve is mechanically connected to the throttle valve, through the link mechanism, such as cable, which is likely to have elongation or backlash. Therefore, the throttle sensing valve is not able to produce the throttle pressure with high accuracy while the throttle opening angle is relatively small, resulting in unfavorable variation in the belt tensioning pressure regulated by the second pressure regulating valve. In view of this variation, the belt tensioning pressure is usually controlled to a level which is higher by a large extra value than a theoretically optimum pressure level. Consequently, the transmission belt is undesirably subject to a large amount of load, causing an accordingly large power loss and low durability of the continuously variable transmission.